An example of a conventional rack-mount data storage system is a disk processor enclosure (DPE) which includes, among other things, a pair of physical storage processors (SPs) and hard drives. The physical SPs process input/output (I/O) requests such as small computer system interface (SCSI) commands from external host computers in order to store host data into the hard drives and load host data from the hard drives on behalf of the external host computers. The hard drives of the DPE may be flash memory drives or magnetic disk drives such as serial attached SCSI (SAS) drives which connect to the pair of SPs via a pair of internal SAS switches.
To increase storage capacity, the operator of the DPE may connect one or more disk-array enclosures (DAEs) containing additional drives to the DPE. One conventional DAE includes (i) a pair of SAS switches which communicate directly with the SPs of the DPE (e.g., link control cards), and (ii) SAS drives. Another conventional DAE includes (i) a pair of peripheral component interconnect express (PCIe) switches which communicate directly with the SPs of the DPE, and (ii) PCIe drives.
To increase connectivity, the operator of the DPE may connect an I/O expansion enclosure which is dedicated to augmenting communications with external equipment. One conventional I/O expansion module includes a pair of PCIe switches which communicate directly with the SPs, as well as front-end I/O modules to connect to external host computers and/or back-end I/O modules to connect to other DAEs.